1. Field of the Invention
The present invention relates to compounds that bind with high affinity and/or specificity to kappa opioid receptors.
2. Background of the Invention
The study of compounds exerting their actions via the opioid receptor system has continued for nearly eight decades. Though this has been a broad effort, the fundamental driving force for this endeavor relates to the elimination or reduction of the side-effect profile produced by the most frequently used or abused opiates morphine (1) and heroin (2) in FIG. 1. Among the many side effects produced by compounds 1 and 2, addiction, tolerance and respiratory depression are of greatest concern when heroin abuse is considered. Though its use waned in the late 70s, increases in both the purity and availability of this drug have promoted a serious resurgence of illegal use. In the study and treatment of substance abuse, antagonists for the opioid receptors like naltrexone (3) (FIG. 1) have played a prominent role. In recent years, researchers studying the physiological mechanisms underlying addiction have sought antagonists selective for each of the three opioid receptor subtypes mu, delta and kappa. Extensive research efforts along these lines lead to the discovery of several such compounds with examples including cyprodime (mu, 4), naltrindole (delta, 5) and nor-binaltorphimine (kappa, 6) (FIG. 1). Of the three, the kappa receptor has only begrudgingly yielded antagonists and, of the known examples, all stem from modification of the prototype, nor-binaltorphimine (nor-BNI, 6).
Portoghese in his pioneering work provided not only the second and third generation kappa antagonists 5′-[(N2-butylamidino)methyl]naltrindole (7) and C5′-guanidinylnaltrindole (GNTI, 8) but also convincing evidence that the Glu297 residue in transmembrane helix 6 of the kappa receptor is the principle address site influencing the kappa selectivity found in 6-8 (FIG. 1). In terms of the message address concept as applied by Portoghese to opioid small-molecules, it is the pendant amine functionality (noted by asterisks in the chart) present in 6-8 that functions as the kappa address element by interacting with the Glu297 residue which is present in the kappa but not in the mu receptor.
In terms of substance abuse treatment, antagonists selective for the kappa receptor have been the least studied primarily due to the limited bio-availability of 6 and its analogs. However, mounting evidence that the endogenous kappa opioid system opposes the actions of mu agonists like 2 suggests that antagonists selective for the kappa receptor system could suppress or eliminate the symptoms of withdrawal which arise from an overactive kappa receptor system and thus could promote abstinence and prevent relapse. Therefore, the development of novel kappa antagonists possessing improved pharmacokinetic profiles would be of great value.
As is obvious from the examples above, the morphinan substructure of 3 has served as the preeminent template upon which selective antagonists have been constructed. Contrary to these efforts, our work in this field started from the relatively unstudied N-substituted trans-(3,4)-dimethyl-4-(3-hydroxyphenyl)piperidine class of opioid antagonist discovered by Zimmerman et al. Compounds like 9a and 9b (FIG. 1) were novel opioid antagonists because their intrinsic antagonist activity was not mediated by the structure of their N-substituent (i.e. the N-methyl (9a) and N-cyclopropylmethyl (9b) analogs in the phenylpiperidine series are both pure antagonists). Indeed, no N-substituent has been discovered which converts this series of compound into an agonist. Compounds 10-12 (FIG. 1) represent some of the structures tried to date. In this connection we recently demonstrated that compounds bearing the trans-cinnamyl N-substituent, as found in 13 (FIG. 1), most closely reproduced the potency at the mu opioid receptor of the flexible N-substituted analogs (10-12). In fact, the comparable mu receptor potencies demonstrated by analogs trans-(3,4)-dimethyl-4-(3-hydroxyphenyl)piperidine possessing the trans-cinnamyl moiety lead us to speculate that in their biologically active conformation, compounds such as 10-12 have the connecting chain and appended ring in their N-substituent extended away from the piperidine nitrogen in a manner consistent with the trans-cinnamyl skeleton like that found in 13.
In more recent studies comparing opioid receptor potency and selectivity to N-substituent changes in this series of antagonists, we discovered 14-21 (FIG. 1). These compounds were obtained from the screening of libraries of compounds which were biased for opioid antagonist activity by incorporation of trans-(3,4)-dimethyl-4-(3-hydroxyphenyl)piperidine into each ligand. In biological testing those compounds (14-21) were found to possess kappa opioid receptor subtype selectivity in binding assays.